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<br />TABlE OF CONTENTS
<br />
<br />APE
<br />ASTM
<br />BMP,
<br />
<br />~
<br />
<br />CEQ
<br />CFR
<br />,r,
<br />CMP
<br />DEP
<br />FA
<br />EO
<br />FEMA
<br />FIRM
<br />NEPA
<br />NHPA
<br />NRHP
<br />NWI
<br />RCRA
<br />SWC
<br />SWD
<br />T&E
<br />USFWS
<br />USACE
<br />
<br />Area of Potential Effect
<br />American Society for Testing Materials
<br />Bel;l Management Practices
<br />Council on Environmental Quality
<br />CodcofFederalRegulations
<br />cubicfcetpersecond
<br />CotTUgatedMetalPipe
<br />Department of Environmental Protection
<br />Environmental Assessment
<br />Executive Order
<br />Federal Emergency Management Agency
<br />Flood Insurance RateMllP
<br />National Environmental Policy Act
<br />National Historic Preservation Act of 1966, as ainended.
<br />National Register of Historic Placcs
<br />NationalWedandlnventory
<br />Resource Conservation and Recovery Act
<br />Swartswood channcry loam, 8 to 15 percclItslopes
<br />Swartswood channery loam, 15 to 25 percent slopes
<br />Thrcatencd & Endangered
<br />UnitcdStatesFishandWildlifcSel"vice
<br />United State.~ Army Corps of Engineers
<br />
<br />'-~
<br />
<br />URS G,../nBT Woodward C/ydB
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<br />SEcnONONE
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<br />Introducuon and Bacllground
<br />
<br />1.1 HISTORICAL FLOODING AND 1997 FLOOD EVENT
<br />
<br />The Pawnee Cn:ck originates in Weld County, Colorado, and flows southeasterly through a
<br />portion of Logan County, before flowing into the South Platte River approximately 1 mile
<br />downstream (northeast) orlhe Town.of Atwood. The Pawn(\(: Creek drainage basin has an area
<br />of 645 square miles. Upon entering the floodplain oflhe South Platte River, the gradient of
<br />Pav.nce Creek becomes quite low.
<br />
<br />Pawnee Creek is documented to have flooded several times vvithin the last 100 years, causing
<br />significant amounts of damages to residential structures, agriculturallands.llIId transportation
<br />(highway and railroad) facilities. Major flooding of Pawnee Creek occurred in 1935, 1965,
<br />1992, and 1997. A vel')' large flood ovent on Pawnee Creek occ;;;rcd (lnring the week of July 28,
<br />1997. Repol1ed 6.hoUf point rainfall depths inthc lltea. ranged from less than I i1J~~toovcr6
<br />incheswithinLoganCountyduringthelulyl997evcnt.
<br />
<br />The flallOpography or this afCa contributes to flooding aCthe Pawnee Creek in the vicinity of
<br />CR 31 andeR 33 with a recurrence interval of once every 10 years. When flooding occurs, split
<br />flows are fanned. Stonnwater on the south side of tho:: ero::ek over tops the: bank and flow
<br />Qvetlau"':!!" the genera] direction of the Town of Atwood. Conversely, tho:: ovctbank flows on the
<br />north side ofti,,:, crc<:k flow overland in the: general direction of the City of Sterling . Asthesplit
<br />flows are fonning, ~.;'r.::- stormwatcr remains in the channel, creating a third, separate flow path.
<br />The split flows contribute to ~.,;p;::al~ !Ioooi..;; situations, different locations., outlined below,
<br />Oenerally, the flooding problems are artributed to OOIlVl.ijlance constrictiotls create dbyroad.
<br />high....'Uy, and/or railroad cmbankments.
<br />
<br />Presently, the Highway 6 and Union Pacific Railroad (UPRR) bridges over Pawnee Creek have
<br />thecapacitytoconveyapproxirnatelyl,800cuble~:etr::tsc::...md(efs). In 1997. the crel'k
<br />channel carried over 6000 cfs ofwatetupstream of the bridges. Water m exccssof the bridges
<br />conveyance capacity is pooled upstream of the bridges and/or pushed north along Highway 6
<br />into the City of Sterling. The water remains pooled upstream of the bridges until the volume of
<br />Pawnee Creek decreasc:s to quantities below 1,800 cfs. At this time, the pooled water can begin
<br />to discharge into the channeL
<br />
<br />OVl'rland flnw nn the north side of Pawnee Creek enters the: northwestern side oflbe city of
<br />Sterling and unites with water from the Sand Creek Drainage as it l10ws through the residential
<br />and business districts of the city on its way to the South Plattc Rivcr. Waterthatl1owsfrom
<br />Pawnee Creek along Highway 6 also enters the western ~ide of the City of Sterling llJld follows
<br />existing drainage systems through residential and business areas of thc ei typriortoflowinginto
<br />tbe South PlalleRiver.
<br />
<br />Flooding in the vicinity of the Town of Atwood oeeurs when the overland flows encounter tbe
<br />UPRR embankment, Ther<: is no eonveY3nee palh to ,,]low the W3lcr to flow pas! the
<br />embankment. As a result, the ovetland flows pond on the upstream side of the embankment,
<br />u"_:_g "c-~:_, ...:_L:_ .L~ '1"_.._ _. \h"~A~ T"A _"""A.-l ,..~,~.,~ "M;~ 'h~ T",.,... "r
<br />~"y"" ",,,...'" "'"'''' ".~ ." "" "', ,,,u,,~. ..~ yv,,~~~ ..~.~, v::;"'..~ H. ~ ~ ,_ .,..,
<br />Atwood until the flow in Pawnee Creek is decreased to a level that allows the ponded water to be
<br /><.hscharged via the tond dr:!mnge system along Highway 6. The cmb;mkrnent w~ washed QUI
<br />and the ponded water llowcd through the breaeb into the SOUd1 I'lalle Rive r,
<br />
<br />URS Greiner Woodward Clyde
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